Oil recovery by steam injection followed by hot water

ABSTRACT

This specification discloses methods of recovering oil from subsurface oil reservoirs penetrated by at least an injection and a production well. Steam is injected into the reservoir to form a steam zone intermediate the injection and production wells. Subsequently hot water at the same temperature as the injected steam is injected into the reservoir to fill the steam zone. Thereafter cold water is injected into the reservoir to drive the hot water toward the production well and oil is recovered from the reservoir via the production well.

O United States Patent n113,572,437

[72] Inventors James E. Marberry 3,193,009 7/1965 Wallace et al. 166/272Calg y, Alberta, Canada; 3,353,598 11/1967 Smith 16 6/245 Henry C.Coutret, Jr, Shreveport, La. 3,421,583 1/1969 Koons 166/269 [211 App].No. 799,177 3,477,510 1 1 1969 Spillette 166/272 [22] Wed 1969 PrimaryExaminer-Stephen J. Novosad [45 1 m 1971 Attorneys-William .1.Scherback, Frederick E. Dumoulin, [731 William D. Jackson, Andrew L.Gaboriault and Sidney A.

. Johnson [54] 011. RECOVERY BY STEAM INJECTION FOLLOWED BY HOT WATER.13 Claims 2 Drawmg Figs ABSTRACT: This specification discloses methodsof recover- US. Cl. oil from subsufface oil reservoirs penetrated atleast an [511 f" E215 43/24 injection and a production well. Steam isinjected into the ofSearch reservoir to form a steam zone intermediatethe injection and 273, 274 production wells. Subsequently hot water atthe same tem- [56] References Cited perature as the injected steam isinjected into the reservoir to fill the steam zone. Thereafter coldwater is injected into the UNITED STATES PATENTS reservoir to drive thehot water toward the production well 3,042,1 14 7/1962 Willman 166/272and oil is recovered from the reservoir via the production well.

Patente c l' Mamh so, 1971 3,512,437

FIG.I

Oil. RECOVERY BY STEAM INJECTION FOLLOWED BY HOT WATER BACKGROUND OF THEINVENTION This invention relates to a thermal method of recovering oilfrom a subsurface oil reservoir. More particularly, this inventionrelates to a method of recovering oil from a subsurface oil reservoirwherein steam is injected into the reservoir through at least oneinjection well and oil is produced therefrom through at least oneproduction well.

Oil recovery from reservoirs is normally characterized either as primaryrecovery or secondary recovery. Primary recovery utilizes the naturallyoccurring forces within the reservoir to force the oil from theformation into production wells. These naturally occurring forcesinclude: (1) the expanding force of high-pressure gas, (2) the buoyantforce of encroaching water, and (3) the force of gravity. Secondaryrecovery utilizes forces applied from extraneous energy sources tosupplement the naturally occurring forces in the reservoir to produceoil therefrom. These secondary forces may result from, for example, gasinjection, steam injection, water injection or in situ combustion. It isnot necessary that the primary forces of the reservoir be exhaustedbefore secondary recovery be initiated. In fact, good reservoirengineering practices many times dictate that secondary recovery bebegun early in the primary recovery cycle though this is sometimescalled pressure maintenance rather than secondary recovery.

The amount of oil recovered by primary means usually varies from l to 50percent of the original oil in place with 15 to 40 percent recoverybeing normal. Therefore more oil usually remains in the reservoir asunrecoverable by primary means than is produced therefrom. Secondaryrecovery is thus extremely important in its application of recovering aportion of this otherwise unrecoverable oil.

A high-pressure steam drive is an example of-a secondary recoveryprocess which is utilized in the recovery of oil. Such a steam driveprocess is described in U.S. Pat. No. 3,353,598, to R. V. Smith. Adepleted reservoir containing a pattern of injection and productionwells is first waterflooded. The waterflood is then terminated and steamis injected into the reservoir. Thereafter steam injection is terminatedand water at normal reservoir temperature is injected into the reservoirto drive the steam and heat through the well pattern to the productionwells. The injection of water at normal reservoir temperature effectssteam condensation at the interface of the water and steam, therebyavoiding later condensation of steam behind the driving front orwater-steam interface. Another steam drive process is described in U.S.Pat. No. 3,360,045, to M. Santourian. This process is concerned withstratum blocking that results when steam is injected into a reservoircontaining heavy crude oil. By this process, a hot, nonaqueous gas isfirst driven through a horizontal zone of the stratum between aninjection and a production well. Steam is then injected into thereservoir for a substantial period of time. Subsequently, it ispreferred to follow the steam with a waterflood drive. The waterfloodmay utilize water at atmospheric temperature or hot water, the latterbeing preferred.

SUMMARY OF THE INVENTION In accordance with the present invention thereis provided a method for recovering oil from a subsurface oil reservoirpenetrated by at least an injection and a production well. Steam isinjected into the reservoir to form a steam zone intermediate theinjection and production wells. Thereafter, hot water at the sametemperature as the injected steam is injected into the reservoir in anamount sufficient to fill the steam zone with this hot water. Oil isrecovered from the reservoir through the production well. In a preferredembodiment of the invention cold water then is injected into thereservoir to drive the hot water through the reservoir toward theproduction well thus further aiding in the recovery of oil from thereser- Another preferred embodiment of this invention is directed torecovering oil from a subsurface oil reservoir penetrated by a patternof injection and production wells. Steam is injected through a firstinjection well into the reservoir to form a steam zone intermediate thefirst injection well and one or more production wells. Subsequently, hotwater of the same temperature as the injected steam is injected throughthe first injection well into the reservoir in an amount sufficient tofill this first steam zone. Upon filling the steam zone, hot waterinjection is terminated and cold water is injected through the firstinjection well into the reservoir to move the hot water toward theproduction well. This process is repeated by selectively using a secondinjection well and a production well which may be the same as ordifferent from the production well used in conjunction with the firstinjection well.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a vertical section of an oilreservoir penetrated by an injection and production well and illustratesa steam zone formed intermediate the wells; and

FIG. 2 is a plan view illustrating a normal five-spot pattern locatedwithin the outlines of an oil reservoir.

DESCRIPTION OF THE PREFERRED EMBODIMENTS This invention is directed to asecondary recovery process for recovering oil from a subsurface oilreservoir penetrated by at least an injection well and a productionwell. Steam is injected into the reservoir to from a steam zoneintermediate the injection and production wells. Thereafter an amount ofhot water at the same temperature as the injected steam is injected intothe reservoir until the steam zone is filled with this hot water. Byinjecting hot water of the same temperature as the steam into the steamzone rather than simply terminating steam injection or injecting coldwater into steam zone, collapse of the steam zone is prevented. Theinjection of cold water into the steam zone or simply terminating steaminjection would result in rapid condensation of the steam and collapseof the steam zone. Such a collapse would result in a lower pressure orpressure sink in the former steam zone and would draw back into thiszone oil which had previously been swept therefrom.

Preferably after hot water injection is terminated, cold water isinjected as a driving fluid into the reservoir to drive the hot waterand heat zone formed from the steam and hot water through the reservoirtoward the production well. The heat of the steam and hot water isthereby efficiently utilized in recovering oil from the reservoir.

More particularly and with reference to FIG. I, there is v shown asubsurface oil reservoir 2 which is penetrated by an injection well 4and a production well 6. The injection well 4 is equipped with tubing 8for the injection of fluids into the reservoir. Production well 6 isequipped with tubing 10 for the production of fluids from the reservoir.Steam 12 from surface equipment (not shown) is injected through tubing 8into reservoir 2 to form a steam zone 14 intermediate the injection andproduction wells. Subsequently, hot water at the same temperature as theinjected steam is injected through tubing 8 into reservoir 2 to fillsteam zone 14. During the process oil is recovered from reservoir 2through tubing 10 of production well 6. Subsequent to the injection ofhot water into reservoir 2 cold water may be injected into the reservoirto move the hot water toward the production well and further facilitateoil recovery from the reservoir through tubing 10 of production well 6.The term cold water" is used in a comparative sense to mean water at atemperature significantly less than the injected steam temperature.Normally the injected cold water will be at ambient temperature, e.g., atemperature in the range of 40 F; to F.

Steam zone 14 is characterized by the existence of a positive steamsaturation. The steam saturation need only be sufficient to permit vaporto flow within it. Such a steam saturation is on the order of 10 to 14percent. The remainder of the pore space within the steam zone is filledwith water and residual oil. The residual oil saturation is relativelylow, typically on the order of 5 to l5 percent. The water in the steamzone is comprised of water originally present in the reservoir, waterformed by partial condensation of the steam, and water injected into thereservoir along with steam. The vapor and liquid within the steam zoneare in thermal equilibrium. Thus, the temperature of the steam zone isdetermined by the pressure within it. The pressure decreases in thesteam zone 14 in going from injection well 4 toward production well 6.Thus the temperature also decreases toward production well 6. Much ofthe pressure drop and consequent temperature decrease occurs in theimmediate vicinity of injection well 4. The remainder of steam zone 14is at a relatively constant temperature which is somewhat lower than theinjection temperature.

As steam 12 is injected into reservoir 2, steam zone 14 is formed havinga steam front 15 as the lead boundary. Steam flows through steam zone 14to steam front 15 where it condenses to form hot condensate which flowsaway from the front. As the steam front moves past a point in thereservoir the oil saturation is reduced to a relatively low value as aresult of several mechanisms. Just ahead of the front, the hotcondensate begins heating the reservoir rock and fluids contained thereby conduction and convection. The boundary between the heated andunheated rock is indicated by boundary 18. The effect of heating the oilis to decrease the viscosity of the oil and improve the oil mobilityratio. Thermal expansion of the oil also occurs, thereby displacing afurther small amount of oil. Movement of the steam front through thispreheated portion of the reservoir causes a further reduction in the oilsaturation through a multiphase flow (gas drive) effect. Behind thesteam front in the steam zone through which steam is flowing the processof steam stripping takes place Little or no liquid oil flow takes placein the steam zone. The stripping process volatilizes the lightercomponents of the oil and reduces the oil saturation to its final lowvalue. The volatilized light components flow with the steam to the steamfront where both materials condense. The light components dilute the oiljust ahead of the steam front and thus form a solvent drive or light oilbank which adds to the effectiveness of the other mechanisms in reducingthe oil saturation.

The steam zone 14 is formed and maintained as the result of steaminjection into reservoir 2. With the start of steam injectioncondensation begins immediately as the steam contacts the cool rock ofthe reservoir. Condensation continues at the boundary of the steam zonewhile this zone exists. Thus, condensation of steam takes place at theupper and lower surfaces of the steam zone and reduces the quantity ofsteam which arrives at steam front 15. A hot water zone 16 is thusformed surrounding steam zone 14. The hot water in hot water zone 16essentially flows parallel to the steam flow toward production well 6.

This invention is directed to a steam injection secondary recoveryprocess which increases the amount of oil recovered from a reservoir perunit volume of injected steam. An amount of steam is injected whichcontains sufficient heat to efficiently produce the oil present within aselected pattern of the reservoir. ln injecting this steam a steam zoneis formed intermediate the injection and production wells. Preferablysteam is injected in an amount less than that resulting in breakthroughof the steam into the production well. In most reservoirs this amount ofsteam forms a steam zone which has an areal sweep within the range of 40to 60 percent of the areal sweep of a steam zone which would exist atbreakthrough of steam into the production well. in most reservoirs thepreferred amount of steam injected into the reservoir varies within therange of 0.25 to 1.0 pore volume measured as water. After the steam zonehas been formed an amount of hot water at the same temperature as thesteam is injected into the reservoir to fill the steam zone. Thisinjection of hot water into the steam zone prevents collapse of thesteam zone and formation of a pressure sink, thereby preventing oilwhich has been swept from the steam zone from being drawn back into it.An

increased amount of oil recovery per unit volume of injected steam isthus achieved.

The preferred quality of the steam injected into the reservoir varieswithin the range of 60 to percent with 80 percent being preferred. Theamount of heat contained by steam increases with the quality of thesteam up to percent quality, i.e., saturated steam. However, with thewaters available for producing steam in an oil field, it is difficult toget higher than about 80 percent quality steam without severedepositional problems within the steam generating equipment. Therefore,80 percent quality steam is normally used in carrying out the invention.

Subsequent to steam injection into the reservoir, hot water at the sametemperature as the injected steam is injected into reservoir 2 in anamount sufiicient to fill steam zone 14. Normally in most reservoirs theamount of hot water to be injected into the reservoir to fill the steamzone varies within the range of 0.02 to 0.03 pore volume. Oil isproduced from reservoir 2 through production well 6 during the steaminjection and hot water injection steps described above. Further, in apreferred embodiment of the invention, cold water is injected intoreservoir 2 subsequent to termination of hot water injection in anamount sufficient to displace the hot water within the reservoir intoproduction well 6. The amount of cold water injected is at least 0.60pore volume and in most reservoirs normally falls within the range of0.60 to l .2 pore volumes.

For reasons previously given the hot water injected into the reservoirshould be at the same temperature as the injected steam to preventcollapse of the steam zone and a resulting pressure sink within thereservoir. Preferably the hot water injected into the reservoir containssteam in an amount of 5 percent or less. This small amount of steam willensure that the hot water is at exactly the same temperature as thepreviously injected'steam. While some latitude can be allowed in thetemperature equivalency between the hot water and the previouslyinjected steam, a difference of only a few degrees Fahrenheit willresult in some condensation in the steam zone with an attendantreduction in pressure. While this can be tolerated in carrying out thepresent invention, it is preferred that the hot water be at exactly thesame temperature as the previously injected steam. This hot water may besupplied simply by applying a sufficient amount of heat to water toraise the temperature to that of the injected steam. On practical way tosupply this hot water is by continuing to utilize surface steamgenerating equipment but apply only enough heat to the boiler water toproduce a fluid which is 5 percent or less quality steam. Anotherpractical means of supplying the hot water for injection into theinjection well is to continue injecting 80 percent quality steam f fromsurface steam generating equipment into the injection well and to mix astream of cold water with the steam in an amount such that the stream ofsteam and water reaching the reservoir is hot water containing 5 percentor less steam.

The hot water may be injected into the reservoir at a rate commensuratewith the capabilities of the hot water generating equipment and thecapabilities of the injection well in the I particular reservoir.However, it is preferred that the hot water be injected at a rate atleast as great as the steam injection rate, measured as liquid. Thisfurther minimizes the pres sure decline at the steam front duringfill-up of the steam zone. A still further preferred embodiment of thisinvention concerns recovering oil from a subsurface oil reservoirpenetrated by a multiplicity of injection wells and one or moreproduction wells. Steam is injected through a first injection well intothe reservoir to form a first steam zone intermediate the firstinjection well and a production well. Subsequently, hot water of thesame temperature as the injected steam is injected into the reservoirthrough the first injection well in an amount sufficient to fill thefirst steam zone with hot water. Cold water then is injected through thefirst injection well into the reservoir to move the hot water toward theproduction well. Upon terminating steam injection in the first injectionwell after forming the first steam zone, steam is injected into a secondinjection well into the reservoir to form a second steam zoneintermediate the second injection well and a production well, which wellmay be the same as or different from the production well used inconjunction with the first injection well. Subsequently, hot water atthe same temperature as the injected steam is injected via the secondinjection well into the reservoir to fill the second steam zone with hotwater. Thereafter, cold water is injected via the second injection wellinto the reservoir to move the hot water of the second steam zone towardthe production well.

This preferred embodiment of the invention is best described byreference to the normal five-spot pattern of FIG. 2. Wells 21, 22, 23,and 24 are injection wells and well 25 is a production well. Steam isfirst injected via injection well 21 to form a first steam zone 26intermediate injection well 21 and production well 25. Preferably steamzone 26 is formed by injecting 0.25 to L pore volume of 60 to 90 percentquality steam measured as water into injection well 21. Thereafter hotwater at the same temperature as the injected steam is injected viainjection well 21 into the pattern in an amount sufiicient to fill thefirst steam zone 26. The amount of hot water injected desirably iswithin the range of 0.02 to 0.03 pore volume. This normally will ensurethat the steam zone is filled with hot water. Thereafter, cold water isinjected via injection well 2! into the pattern to displace the hotwater toward production well 25. Preferably cold water in the amount ofat least 0.60 pore volume is injected through the injection well intothe reservoir. Subsequent to the injection of steam into the injectionwell 21 steam is injected via injection well 22 into the pattern to forma second steam zone 28 intermediate injection well 22 and productionwell 25. Again, about 0.25 to about 1.0 pore volume of 60 to 90 percentquality steam measured as water may be injected via injection well 22 toform second steam zone 28. Thereafter, hot water in the amount of 0.02to 0.03 pore volume may be injected via injection well 22 to fill thesecond steam zone. Subsequently at least 0.60 pore volume of cold wateris injected via injection well 22 to move the hot water towardproduction well 25. This procedure is then repeated utilizing injectionwell 23 and production well 25 and thereafter utilizing injection well24 and production well 25.

While this invention has been described with particular reference to thenormal five-spot pattern of FIG. 2, it is of course understood that itis applicable to other patterns as well. For example, it may be employedin a direct line drive wherein injection wells are formed in a line andproduction wells are formed in another line with each injection wellbeing directly offset by a production well. In such a direct line drivethere is realized the nearest approach to a complete vertical planaradvance of the flooding medium. A modification of the direct line driveis the staggered line drive wherein production wells are diagonallyoffset from injection wells. A developed five-spot pattern is a case ofthe staggered line drive wherein the distance between all like wells isconstant.

In addition, this invention is applicable with normal fourspot,seven-spot, and nine-spot patterns. The normal four-spot pattern employsthree injection wells surrounding one production well; a normalseven-spot pattern employs six injection wells surrounding a productionwell; and a normal nine-spot pattern consists of eight injection wellssurrounding one production well. This invention is further applicable toinverted patterns. For example, it is applicable to an invertednine-spot pattern which has eight production wells that surround oninjection well.

As used herein pore volume means pattern pore volume or, in other words,the pore space within a reservoir encompassed by a particular pattern.Pore volume is normally expressed in accordance with equation l below:

where:

P. V. pattern pore volume of the reservoir; A area of the pattern;

h thickness of the reservoir; and I porosity of the reservoir. In caseof a two-well pattern consisting of an injection and a production well,the pattern is generally considered elliptical in shape. In such atwo-well pattern the area A may be calculated by squaring the distancebetween injection and production wells.

We claim: 1. Amethod of recovering oil from a subsurface oil reservoirpenetrated by an injection well and a production well,

comprising the steps of:

a. injecting steam via said injection well into said reservoir to form asteam zone intermediate said injection well and said production well;

b. subsequently injecting hot water via said injection well into saidreservoir in an amount sufficient to fill said steam zone, said hotwater being at the same temperature as said injected steam of step (a);and

c. recovering oil from said reservoir through said production well.

2. A method of recovering oil from a subsurface oil reservoir penetratedby an injection well and a production well, comprising the steps of:

a. injecting steam via said injection well into said reservoir to form asteam zone intermediate said injection well and said production well; i

b. subsequently injecting hot water via said injection well into saidreservoir in an amount sufficient to fill said steam zone, said hotwater being at the same temperature as said injected steam of step (a);

c. subsequently injecting cold water via said injection well into saidreservoir to move said hot water toward said production well; and

d. recovering oil from said reservoir through said production well.

3. The method of claim 2 wherein said injected steam is of a qualitywithin the range of 60 to 90 percent.

4. The method of claim 2 wherein said injected hot water contains steamin an amount of not more than 5 percent.

5. The method of claim 2 wherein said hot water is injected at a rate atleast as great as the steam injection rate.

6. A method of recovering oil from a subsurface oil reservoir penetratedby an injection well and a production well, comprising the steps of:

a. injecting steam via said injection well into said reservoir to form asteam zone having an areal sweep of a size less thanthe size of theareal sweep which would exist at steam breakthrough into said productionwell;

b. subsequently injecting hot water via said injection well into saidreservoir in an amount sufficient to fill said steam zone, said hotwater being at the same temperature as said injected steam of step (a);

c. injecting cold water via said injection well into said reser voir tomove said hot water toward said production well; and

d. producing oil from said reservoir through said production well.production 7. The method of claim 6 wherein said areal sweep of saidsteam zone is within the range of 40 to 60 percent of the areal sweep ofa steam zone which would exist at breakthrough of steam into saidproduction well.

8. A method of recovering oil from a subsurface oil reservoir penetratedby an injection well and a production well, comprising the steps of:

a. injecting through said injection well into said reservoir 0.25 to 1.0pore volume of 60 to '90 percent quality steam thereby forming a steamzone intermediate said injection well and said production well;

b. subsequently injecting into said reservoir through said injectionwell 0.02 to 0.03 pore volume of hot water of the same temperature assaid injected steam of step (a) thereby filling said steam zone with hotwater;

c. subsequently injecting into said reservoir through said injectionwell at least 0.60 pore volume of cold water; and

d. recovering oil from said reservoir through said production well.

9. A method of recovering oil from a subsurface oil reservoir penetratedby a multiplicity of injection wells and production wells, comprising:

a. injecting steam through a first injection well into said reservoir toform a first steam zone intermediate said first injection well and aproduction well;

b. subsequent to step (a) injecting through said first injection wellhot water of the same temperature as said injected steam into saidreservoir in an amount sufficient to fill said first steam zone withsaid hot water;

c. subsequent to step (b) injecting cold water through said firstinjection well into said reservoir to move said hot water toward saidproduction well;

d. subsequent to step (b) injecting steam through a second injectionwell into said reservoir to form a second steam zone intermediate saidsecond injection well and a production well;

e. subsequent to step (d) injecting hot water at the same temperature assaid injected steam into said oil reservoir to fill said second steamzone with said hot water; and

f. subsequent to step (e) injecting cold water through said secondinjection well into said oil reservoir to move said hot water in saidsecond steam zone toward said production well.

10. The method of claim 9 wherein 0.25 to 1.0 pore volume of 60 topercent quality steam is injected through each of said first and secondinjection wells.

11. The method of claim 10 wherein 0.02 to 0.03 pore volume of hot waterat the same temperature as said injected steam is injected through eachof said first and second injection wells.

12. The method of claim 11 wherein at least 0.60 pore volume of coldwater is injected through each of said first and second injection wells.

13. A method of recovering oil from a subsurface oil reservoirpenetrated by an injection well and a production well. comprising thesteps of: v

a. injecting steam via said injection well into said reservoir to form asteam zone having an area] sweep within the range of 40 to 60 percent ofthe areal sweep of a steam zone which would exist at breakthrough ofsteam into said production well;

b. subsequently injecting hot water via said injection well. into saidreservoir in an amount sufficient to fill said.

steam zone, said hot water being at the same temperature as saidinjected steam of step (a); and

c. recovering oil from said reservoir through said production well.

2. A method of recovering oil from a subsurface oil reservoir penetratedby an injection well and a production well, comprising the steps of: a.injecting steam via said injection well into said reservoir to form asteam zone intermediate said injection well and said production well; b.subsequentlY injecting hot water via said injection well into saidreservoir in an amount sufficient to fill said steam zone, said hotwater being at the same temperature as said injected steam of step (a);c. subsequently injecting cold water via said injection well into saidreservoir to move said hot water toward said production well; and d.recovering oil from said reservoir through said production well.
 3. Themethod of claim 2 wherein said injected steam is of a quality within therange of 60 to 90 percent.
 4. The method of claim 2 wherein saidinjected hot water contains steam in an amount of not more than 5percent.
 5. The method of claim 2 wherein said hot water is injected ata rate at least as great as the steam injection rate.
 6. A method ofrecovering oil from a subsurface oil reservoir penetrated by aninjection well and a production well, comprising the steps of: a.injecting steam via said injection well into said reservoir to form asteam zone having an areal sweep of a size less than the size of theareal sweep which would exist at steam breakthrough into said productionwell; b. subsequently injecting hot water via said injection well intosaid reservoir in an amount sufficient to fill said steam zone, said hotwater being at the same temperature as said injected steam of step (a);c. injecting cold water via said injection well into said reservoir tomove said hot water toward said production well; and d. producing oilfrom said reservoir through said production well. production
 7. Themethod of claim 6 wherein said areal sweep of said steam zone is withinthe range of 40 to 60 percent of the areal sweep of a steam zone whichwould exist at breakthrough of steam into said production well.
 8. Amethod of recovering oil from a subsurface oil reservoir penetrated byan injection well and a production well, comprising the steps of: a.injecting through said injection well into said reservoir 0.25 to 1.0pore volume of 60 to 90 percent quality steam thereby forming a steamzone intermediate said injection well and said production well; b.subsequently injecting into said reservoir through said injection well0.02 to 0.03 pore volume of hot water of the same temperature as saidinjected steam of step (a) thereby filling said steam zone with hotwater; c. subsequently injecting into said reservoir through saidinjection well at least 0.60 pore volume of cold water; and d.recovering oil from said reservoir through said production well.
 9. Amethod of recovering oil from a subsurface oil reservoir penetrated by amultiplicity of injection wells and production wells, comprising: a.injecting steam through a first injection well into said reservoir toform a first steam zone intermediate said first injection well and aproduction well; b. subsequent to step (a) injecting through said firstinjection well hot water of the same temperature as said injected steaminto said reservoir in an amount sufficient to fill said first steamzone with said hot water; c. subsequent to step (b) injecting cold waterthrough said first injection well into said reservoir to move said hotwater toward said production well; d. subsequent to step (b) injectingsteam through a second injection well into said reservoir to form asecond steam zone intermediate said second injection well and aproduction well; e. subsequent to step (d) injecting hot water at thesame temperature as said injected steam into said oil reservoir to fillsaid second steam zone with said hot water; and f. subsequent to step(e) injecting cold water through said second injection well into saidoil reservoir to move said hot water in said second steam zone towardsaid production well.
 10. The method of claim 9 wherein 0.25 to 1.0 porevolume of 60 to 90 percent quality steam is injected through each ofsaid first and seconD injection wells.
 11. The method of claim 10wherein 0.02 to 0.03 pore volume of hot water at the same temperature assaid injected steam is injected through each of said first and secondinjection wells.
 12. The method of claim 11 wherein at least 0.60 porevolume of cold water is injected through each of said first and secondinjection wells.
 13. A method of recovering oil from a subsurface oilreservoir penetrated by an injection well and a production well,comprising the steps of: a. injecting steam via said injection well intosaid reservoir to form a steam zone having an areal sweep within therange of 40 to 60 percent of the areal sweep of a steam zone which wouldexist at breakthrough of steam into said production well; b.subsequently injecting hot water via said injection well into saidreservoir in an amount sufficient to fill said steam zone, said hotwater being at the same temperature as said injected steam of step (a);and c. recovering oil from said reservoir through said production well.